Several liquid crystal driving modes have been proposed and developed to improve its display performance. The control of certain pretilt angles according to the driving modes is needed, and an alignment film that generates pretilt angles in a stable way becomes an essential element in determining the performance of LCD. Basically, alignment films align liquid crystal molecules, and they form pretilt angles between substrate planes and liquid crystal molecules so that they make reactivity good and ensure the alignment stability of liquid crystal molecules. Also, the alignment film is an important factor in determining credibility, display uniformity, after-image and voltage-holding ratio, etc. in liquid crystal display devices.
Numerous materials ranging from inorganic materials to organic polymers have been applied as materials for alignment films. Of them, the mostly-used representative polymer compounds are soluble polyimide based polymer compositions and polyamide acid based polymer compositions using polyamide acids through imidization. They are widely used as alignment materials that align liquid crystal in industrial fields by virtue of their excellent heat resistance and chemistry resistance. These polymer compounds are formed by the polymerization of diamine and tetracarboxylic acid dianhydrides, where the structure of monomers determines the properties of polymer compounds synthesized therefrom.
In general, as means for obtaining high pretilt angles, side-chained polyimide compounds have been used. Side chain structure can be introduced into either diamine or tetracarboxylic acid dianhydride but in most cases, diamine is used because side chain groups can be easily introduced thereinto. In general, there have been known polyimide liquid crystal alignment materials using as a monomer, an aliphatic, side-chained diamine having a straight alkoxy group, alkyl ester group or fluorinated alkyl group as a side chain. However, as the distribution and length of the side chains are not controlled in such polyimide alignment films, their pretilt angles were often as low as 3˜25 degree or so.
In the side-chained polyimides, the several characteristics of the alignment films are determined according to aromatic and aliphatic components. Since the aromatic components act as a rigid core in polymer chains, they result in low solubility in organic solvents and thus decrease processability in industry. In addition, as they make the formation of charge transfer complex between polymer chains easy, they cause low permeability in the visible ray region. In particular, in TFT-type liquid crystal display devices, voltage holding ratio acts as an important factor in display characteristics of liquid crystal display devices and alignment films containing aromatic components in large amounts readily absorb ions so that they decrease applied voltage, resulting in decrease in contrast.
On the contrary, alignment films containing aliphatic or alicyclic classes in large amounts can improve the above-mentioned drawbacks. However, aliphatic polyamide acid based alignment materials have poor alignment properties of liquid crystal and aliphatic soluble polyimides have reduced adhesion to substrates so that even weak rubbing can readily chip off coating membranes. Although there are alignment materials where polyimides and polyamic acids are blended to complement such drawbacks, they have problems that the two materials are separated by heat and in particular, liquid crystal alignment materials prepared by the block copolymerization of the polyimides and polyamic acids involve too complicated preparation routes.